Does anyone have a Maltese with skin allergies????

I have an 8 month old Maltese and she started chewing, scratching and pulling the hair off her one leg. The vet said to give Benadryl. That worked for awhile but now she started again. She had some scabs on her head and on the side of her face from scratching. Now the vet thinks it could be from her food. I had her on Royal Canin puppy food but after she got spayed the vet said to give her adult food so I got Royal Canin Special 30. She has been on it for about 3 weeks now. Would it be the food? He gave me Annamaet but she is not fond of it. Is it possible to get another good food and mix it without making her sick? I would get one that is also good for dogs with allergies. Can anyone help? I don't want her to have allergies all her life so I would like to correct it now if it is from her food.